This invention pertains to cyclone separators, equipped with a plurality of closed-bottom cyclones each having solids discharge means and gas reflux means, used to recover catalyst fines in the fluidized catalytic cracking of heavy hydrocarbon feeds. More particularly, the invention relates to the discovery and inhibition of cross talk between the solids discharge means and the gas reflux means.
Cyclone separation is used to separate particles from gas. A particulate-laden gas stream is introduced into a cylindrical chamber. A spin is imposed on the gas, either by tangential addition or axial addition coupled with swirl vanes. Solids are thrown to the wall of the chamber by inertia, while a cleaner gas is withdrawn from a central region of the cyclone. Some gas exits with the solids. Solids may be discharged via a tangential outlet through a sidewall of the cyclone, or axially discharged.
There are many types of cyclone separators, but they can be arbitrarily classified as open-bottom or closed-bottom. The present invention is especially useful for improving the operation of closed-bottom cyclones. Closed-bottom cyclones have a generally cylindrical body which is essentially closed save for one or more feed inlets, having usually only one gas outlet and a solids outlet. Closed-bottom cyclones are effectively isolated from the atmosphere of a vessel containing the cyclone. Closed-bottom cyclones usually run at a slightly higher or slightly lower pressure than the atmosphere in the vessel around the cyclone. If pressure in the cyclone body is higher than the pressure outside the cyclone, then the cyclone is a positive pressure cyclone. If pressure in the cyclone body is lower than the pressure outside the cyclone then the cyclone is a negative pressure cyclone.
In a closed-bottom cyclone the feed gas is generally added tangentially to an end portion of the cyclone body. The gas outlet is usually a tube, axially aligned with the longitudinal axis of the cyclone body, passing through the same end of the cyclone receiving feed gas. The solids are usually withdrawn via an elongated dipleg at an end of the cyclone body opposing the gas outlet. In a third stage separator (TSS), discussed below, the solids are sometimes withdrawn via a horizontal slit or slot in the wall of the cyclone body, and usually at an end opposite from the gas outlet.
Somewhat related to the distinction between open- and closed-bottom cyclones is whether or not the cyclone dust outlet shares the same space (or vessel volume) as the gas inlet or is isolated. When cyclones receive feed gas from, and discharge into, a fluid bed like the fluid catalytic cracking (FCC) regenerator, gas discharged with solids through the diplegs recirculates up from the fluid bed to the cyclone inlet that is in the same space. Gas recycle from solids outlet to gas feed inlet occurs because the solids outlet and gas feed inlet share the same vapor volume in the reactor. When the feed gas inlet is fluidly isolated from the solids outlet, this type of gas recycle does not take place. When solids discharge into a closed vessel such as the bottom of a TSS, gas discharged with solids through the dust outlet of the cyclone cannot return to the cyclone inlet. It was always assumed that because the dust catcher was essentially sealed that no more gas would escape via the solids outlet than would be removed with the solids phase removed from the dust catcher, until the innovation to use a gas reflux in the outlet end of the cyclone as described in U.S. Pat. No. 5,681,450 to Chitnis et al., which is hereby incorporated in its entirety for all purposes by reference herein.
Briefly, the problem solved by the Chitnis et al. patent can be mentioned now as follows. In some types of cyclones, and in some cyclone placements, vapor recycle from the solids outlet to the feed gas inlet is not a significant problem. In open-bottom cyclones, pressures are essentially in balance inside and outside the cyclone. Some gas always leaves with or is entrained or aspirated with departing solids flow. Such gas, beyond that which is eventually removed with the separated solids, easily returns into the open body of the cyclone. In closed- or open-bottom cyclones where the dust outlet shares the same vapor space as the cyclone feed gas inlet, gas recycle is never a problem. The problem appears in generally closed cyclones where gas discharged with solids from the device has no easy way back into the cyclone, but the problem can appear to some extent even in open-bottom cyclones.
In fluid catalytic cracking, catalyst having a particle size and color resembling beach sand circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425° C. to 600° C., usually 460° C. to 560° C. The cracking reaction deposits coke on the catalyst, thereby deactivating it. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen-containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500° C. to 900° C., usually 600° C. to 750° C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
One failure mode in FCC is erosion of the cyclone caused by years of operation with high velocity, catalyst fines-laden gas passing through the cyclones. Refiners do not like to use high internal cyclone velocities, but are driven to them because of the need to improve cyclone efficiency. The FCC unit must operate without exceeding local emission limits on particulates. The catalyst is somewhat expensive, and most units have over a hundred tons of catalyst in inventory. FCC units circulate tons of catalyst per minute. Large circulation rates are needed because feed rates are large, and for every ton of oil cracked, roughly 5 tons of catalyst are needed.
These large amounts of catalyst must be removed from cracked products lest the heavy hydrocarbon products be contaminated with catalyst fines. Even with several stages of cyclone separation some catalyst fines invariably remain with the cracked products. These concentrate in the heaviest product fractions, usually in the main FCC fractionator bottoms, sometimes called the slurry oil because so much catalyst is present. Refiners may let this material sit in a tank to allow some entrained catalyst to drop out, producing CSO or clarified slurry oil.
The problems are as severe or even worse in the regenerator. In addition to the large amounts of catalyst circulation needed to satisfy the demands of the cracking reactor, there is an additional internal catalyst circulation that must be dealt with. In most bubbling bed catalyst regenerators, an amount of catalyst equal to the entire catalyst inventory will pass through the regenerator cyclones every 15-30 minutes. Most units have several hundred tons of catalyst inventory. Any catalyst not recovered using the regenerator cyclones, typically comprising two stages of cyclones, will remain with the regenerator flue gas, unless a third stage separator, electrostatic precipitator or some sort of removal stage is added at considerable cost.
Many refiners use a power recovery system. The energy in FCC regenerator flue gas drives the air blower supplying air to the regenerator. The amount and particle size of fines in most FCC flue gas streams exiting the regenerator is enough to erode turbine blades if a power recovery system is installed. Generally a third stage separator unit is installed upstream of the turbine to reduce the catalyst loading and protect the turbine blades, especially from particles greater than 10 μm in size. Some refiners even now install electrostatic precipitators or some other particulate removal stage downstream of third stage separators and turbines to further reduce fines emissions.
Many refiners now use high efficiency third stage cyclones to decrease loss of FCC catalyst fines and/or protect power recovery turbine blades. It should be mentioned that whenever a third stage separator is used to clean up regenerator flue gas a fourth stage separator is typically used to process the underflow (solids rich portion) discharged from the third stage separator. Gas volumes in the fourth stage separator are small because third stage cyclone designs minimize the amount of gas discharged with the solids. Typically 0.5 to 3% of the flue gas is removed with the solids discharged from the third stage separator. Third stage separators limit gas discharged with solids (gas in the underflow) to that needed to fluidize and discharge solids from the third stage separator.
For these reasons, small size particles and relatively low gas volumes, the fourth stage separator generally comprises small diameter cyclones, a hot sintered metal or ceramic filter, or a bag house.
Most refiners are satisfied with their primary and secondary cyclones or equivalent means for recovering catalyst from flue gas and discharging recovered catalyst back into the regenerator. The troublesome separation is downstream of the regenerator in the third stage separator or TSS unit. The TSS must produce gas with essentially no particles greater than 10 microns (when power recovery turbines are used) and/or achieve sufficient removal of fines to meet emissions particulates regulatory limits.
Modern, high efficiency third stage separators typically have 50 to 100 or more small diameter cyclones. One type of third stage separator is described in “Improved Hot-Gas Expanders For Cat Cracker Flue Gas,” Hydrocarbon Processing, March 1976. The device is fairly large, a 26-ft diameter vessel. Catalyst laden flue gas passes through many swirl tubes. Catalyst is thrown against the tube walls by centrifugal force. Clean gas is withdrawn up via a central gas outlet tube while solids are discharged through two blowdown slots in the base of an outer tube. The device removed most 10 micron and larger particles. The unit processed about 550,000 lbs/hour of flue gas containing 300 lbs/hour of particles ranging from sub-micron fines to 60 micron sized catalyst particles. This corresponds to an inlet loading of about 680 mg/Nm3.
The solids loading on various cyclones in various parts of the FCC process varies greatly. The third stage separator has the most difficult separation in terms of particle size, while the primary separators typically do 99% of the solids recovery. This can be put into perspective by considering what happens in an exemplary FCC unit. In this exemplary unit the separation progresses as shown below in Table A, going from the primary and secondary cyclones making the initial separation of catalyst from flue gas in the regenerator through the TSS as final separation for the gas stream. The separated dust from the TSS is withdrawn to the fourth stage. Gas from the fourth stages merges with that of the third stage. Total emissions are 0.0474 tons/hr, corresponding to a loading of 215 mg/Nm3.
TABLE ACYCLONE STAGE1st2nd3rd4thCYCLONE INLETTons/hr solid5005.00.150.108Average particle size,65401317micronsTons/hr gas2752752758Weight gas/weight solids0.5555183374CYCLONE GASOUTLETTons/hr solid5.00.150.0420.0054Average particle size,40131.55micronsTons/hr gas2752752678% gas via gas outlet10010097100% solids removal/stage99977295CYCLONE SOLIDSOUTLETTons/hr solid4954.850.1080.1026Average particle size,65411718micronsTons/hr gas˜0˜08˜0% gas via solids outlet˜0˜03˜0weight gas/weight solids˜0˜074˜0
The total solids throughput per cyclone is expressed as tons per hour. The average particle size of the solids feed to each cyclone changes markedly. Larger particles are preferentially removed so each downstream stage, through the third stage, sees fewer solids with a much smaller particle size distribution.
The first stage or primary cyclones do most of the work, generally recovering more than 99% of the total solids in a single stage. The first stage cyclones also have the easiest job, in that the particles are relatively large, around 60-80 microns, there are large amounts of gas available to generate centrifugal forces, and the discharge of significant amounts of flue gas down the cyclone dipleg has no adverse consequences. The solids loading is high. The first stage cyclone removes large amounts of solids using large amounts of gas to generate centrifugal forces. Some gas is discharged with the solids, as gas is needed to maintain fluidization of the recovered solids. This entrained gas simply recycles through the bed of catalyst in the regenerator and returns into the inlet horn of the first stage or primary cyclone.
The secondary FCC regenerator cyclones treat as much gas as the primary cyclones, but orders of magnitude less solids. Secondary cyclones recover typically around 95-98% of the solids charged to them. The secondary cyclones can recover additional amounts of particulates from gas discharged from the primary cyclones. This is because lower solids loadings in the secondary cyclones permit higher gas velocities to be used in secondary cyclones than in primary cyclones. Higher gas velocities develop higher centrifugal forces to improve efficiency. The second stage cyclone has about the same gas flow as the first, but orders of magnitude less solids. Small amounts of solids are discharged from the second stage cyclone diplegs, with small amounts of gas. As is the case in the first stage, this gas simply reenters the FCC regenerator atmosphere. This gas becomes a small part of the gas feed to the first stage cyclone.
Because primary and secondary cyclones are so efficient, essentially all of the easy-to-remove particles are removed after two stages of cyclone separation. Only fines, irregularly shaped fragments of FCC catalyst, remain in the gas to the third stage separator. Third stage separator cyclone operation is characterized by large volumes of gas and small amounts of extremely fine particulates, much of it smaller than 5 microns. Each downstream stage, from the first stage through the third stage, sees less solids and smaller particles. Much more gas per unit weight of solids is discharged via the solids outlet in third stage separators as compared to cyclones in the regenerator. On a weight basis, more than 10 grams of gas are discharged per 1 gram of fines discharged to the catch chamber of TSS cyclones. Contrast this with operation in first and second stage cyclones in the FCC regenerator, where 1 gram of gas transports over 1000 grams of FCC catalyst down a dipleg. It can be said that from 10,000 to 100,000 times as much gas is present in third stage separator solids discharge stream as compared to solids discharge via the dipleg of a primary cyclone. In part because of the relatively small amounts of solids involved, and large gas volumes, typically 5 orders of magnitude more gas relative to solids, many TSS cyclones are open-bottom.
After the gas leaves the gas outlet of the third stage separator, it has a sharply reduced solids content as compared to say the gas from the first stage cyclone. Although there is not much solids loading at this point, the amount of solids may be sufficient to destroy or damage power recovery turbines, and may exceed local existing or projected limits on particulates emissions, which at several localities are as low as 50 mg/Nm3.
These developments are somewhat surprising in view of the many improvements which have occurred both in cyclone efficiency and catalyst properties. Cyclone efficiency has improved during the 50 years FCC has been in use in refineries. FCC catalysts are stronger and more attrition resistant. These factors (better cyclones, stronger catalyst), if considered alone, would make FCC flue gas cleaner. Offsetting factors have included an increase in catalyst circulation rates, multiplying the load on the cyclones. Higher cyclone efficiencies are achieved by using higher velocities in the cyclones to generate stronger centrifugal forces. The high velocities can fracture or break even modern attrition-resistant catalysts to produce more fines which are harder to recover and also tend to wear out the cyclones. High efficiency (and high velocity) cyclones increase fines recovery, but the gas makes more turns in the cyclone body, increasing catalyst attrition.
Particle recovery in conventional large diameter cyclones associated with the FCC regenerator had reached a plateau. Refiners resorted to third stage separators (TSS) with many small diameter cyclones to increase particulate removal from FCC flue gas. Mechanically, third stage separators are complex. Many TSS cyclones are needed to handle the large volumes encountered in FCC flue gas streams. Each cyclone is of small diameter and is mounted either vertically or horizontally. One cyclone manufacturer uses many small cyclones, 10 inches in diameter, to increase centrifugal forces and reduce radial distance to a wall where solids could collect. Since many cyclones are needed, it is generally necessary to install them in a single vessel, acting as a manifold. The TSS unit made it easier for particles to reach the wall of the cyclone by reducing the distance to the cyclone walls. The offsetting factor is some increase in pressure drop, and considerable capital expense for a unit which made only a modest improvement in fines removal.
TSS units allowed refiners to reach a new level of solids recovery, but known TSS units were not always adequate for refiners wanting to use power recovery turbines. Refiners were at an impasse for improving TSS cyclone efficiency. Then, the innovations of Chitnis et al. (U.S. Pat. No. 5,681,450) improved the operation of cyclones, especially their performance on the less than 5 micron particles, which are difficult to remove in conventional cyclones and difficult and costly to remove using electrostatic precipitation. These improvements reduced dust re-entrainment from TSS units. TSS cyclones had not satisfactorily addressed the problem of how to deal with gas discharged with the solids. Relatively large amounts of gas are discharged with solids whether solids are discharged tangentially via a slot cut in a sidewall of the cyclone barrel or axially via an open bottom solids outlet opposite the clean gas outlet. In some TSS cyclones there is no dipleg and no place for de-aeration of solids. The solids are discharged at a relatively high velocity and may aspirate a significant amount of gas. Rather than gas fluidizing the solids, the solids are carrying out excessive amounts of gas. At the solids outlet of TSS cyclones, the considerable kinetic energy in the solids carries gas from the cyclone body. Chitnis et al. wondered what happens to this gas, and observed significant and fluctuating flow of gas out of and into the longitudinal slot of test cyclones, which seemed chaotic. Gas from one cyclone flowed into the catch chamber and then reentered the same cyclone as well as other cyclones. Reducing length and width of the slot seemed to reduce the gas flow and the interaction between the cyclones.
Chitnis et al. realized the discharged catalyst always carried gas with it into the catch chamber where it had no place to go. Only a minor amount of this gas was needed for the underflow. Much larger amounts of gas exited the cyclone, and re-entered the cyclone in some way. Chitnis et al. observed localized pulses of catalyst/gas discharge alternating with reverse flow of gas back into the barrel slot. This was the only way for gas discharged with the solids-rich phase to return to the cyclones from the catch chamber, as there was not enough gas removed with the solids phase from the bottom of the device to mass balance the cyclones. Chitnis et al. concluded that all TSS cyclones unintentionally recycle or reflux gas back through the solids outlet into the cyclone body. Refluxing gas entrained solids back into the inner vortex of the cyclone and out the gas outlet tube. The problem stemmed from the way the gas returns into the cyclone body of the cyclone. Without some other means for gas reflux, a reverse flow, chaotic or randomly fluctuating in space and time, refluxed this gas via the solids outlet. This gas reflux flow was countercurrent to flow out of the solids discharge. A cyclone could alternate between a high solids discharge phase and a lower solids reflux phase. Another alternative was for one end of an outlet slot to discharge solids while the other end of the slot permitted reflux back into the cyclone. In either case the effect of chaotic reflux via the solids outlet was to disrupt gas streamlines of the tangential flow within the cyclone body.
Chitnis et al. Week able to stabilize the cyclone operation, and roughly halve the amount of fines discharged from the gas outlet tube, by providing a separate gas reflux means, preferably in the bottom of the cyclone body. Still, further improvements in the solids separation in the Chitnis et al. third stage separator cyclones were desired.